The present invention relates to a CATV (cable television) system, and more particularly to an unauthorized channel jamming signal applying method in a CATV system.
A device for applying a jamming signal to a designated channel, namely, a channel for which viewing is not authorized to a subscriber, in a CATV system which uses cable to transmit video signals is well known in the art. (See, for instance, Japanese Laid-Open Patent Applications Nos. 153338/1984 and 193390/1984). An example of such a conventional unauthorized channel jamming signal applying device is shown in FIG. 1. In FIG. 1, reference numeral 1 designates a main cable line in a CATV system which transmits multi-channel television signals; and 2, the jamming signal applying device. The output of the jamming signal applying device is formed by mixing a jamming signal and a video signal in a mixer, and is applied through output terminals 3, 4, 5 and 6 and respective drop lines to terminal units in CATV subscribers' homes. The multi-channel TV signals transmitted through the main cable line 1 are distributed, for instance, at four points by a distributor 7.
The jamming signal applying device includes a jamming signal generating unit 8 implemented with a voltage-controlled oscillator. The jamming signal generating unit 8 provides an output in response to a control signal voltage from a controller 9. The output voltage of the controller 9 controls the jamming signal's frequency. The controller 9 also controls the on-off operation of RF switches 11, 12, 13 and 14 when a jamming signal produced by the jamming signal generating unit 8 is supplied, via a distributor 10, through the RF switches to the respective output terminals. Specifically, the controller 9 opens the RF switches for the period of time that the jamming is provided for contracted (authorized) channels to prevent the application of the jamming signal to the contracted channels, and the RF switches are closed for the period of time that the jamming signal is provided for noncontracted (unauthorized) channels to allow the application of the jamming signal to the noncontracted channels. Thus, the TV signals provided at the output terminals are scrambled (jammed) or not depending on whether the channels are contracted ones or not.
In the above-described case, the jamming signal is distributed to four points; however, the number of output terminals and the number of RF switches can be increased according to the number of subscribers.
If, in order to obtain the highest degree of scrambling of the picture, the period T of applying the jamming signal is set to a value which is an integer times the horizontal synchronizing frequency, the picture will be strongly affected, but the sound is scarcely scrambled. Accordingly, if this jamming signal applying method is employed, it is necessary to produce another jamming signal to scramble the sound, or else to change the jamming signal so as to better scramble the sound but with the degree of scrambling of the picture decreased. This will be described in more detail.
In the above-described jamming signal applying device, the voltage (or jamming signal frequency) applied to the jamming signal generating unit 8 changes with time as shown in FIG. 2. In FIG. 2, the stair-shaped stages represent jamming channel frequencies (A through G), reference character T designates the period of application of the jamming signal to a channel, and t, the time for which the jamming signal is applied. In the case of FIG. 2, for seven channels to be scrambled, T/t=7, and the time sharing period is constant. It should be noted that FIG. 2 shows the operation of one of the RF switches, namely, where the jamming signal is applied to the channel C only. That is, only when the jamming signal frequency is for the channel C is the respective RF switch closed; when the jamming signal frequency is not for the channel C, the RF switch is opened to prevent the application of the jamming signal.
The degree of scrambling of the picture on the subscriber's television set depends on the values of T and t. In general, a higher degree of scrambling is obtained by setting T to a value which is an integer times the horizontal synchronizing frequency of the video signal. On the other hand, t is defined only by T and the number of channels to be jammed.